MgO-based tunnel spin injectors

ABSTRACT

A MgO tunnel barrier is sandwiched between semiconductor material on one side and a ferri- and/or ferromagnetic material on the other side to form a spintronic element. The semiconductor material may include GaAs, for example. The spintronic element may be used as a spin injection device by injecting charge carriers from the magnetic material into the MgO tunnel barrier and then into the semiconductor. Similarly, the spintronic element may be used as a detector or analyzer of spin-polarized charge carriers by flowing charge carriers from the surface of the semiconducting layer through the MgO tunnel barrier and into the (ferri- or ferro-) magnetic material, which then acts as a detector. The MgO tunnel barrier is preferably formed by forming a Mg layer on an underlayer (e.g., a ferromagnetic layer), and then directing additional Mg, in the presence of oxygen, towards the underlayer.

This application is a continuation of, and claims priority to,Applicant's copending application Ser. No. 10/969,735 filed Oct. 19,2004 and entitled “MgO-Based Tunnel Spin Injectors”, which in turn is acontinuation-in-part of, and claims priority to, Applicant's copendingapplication Ser. No. 10/646,247 filed Aug. 22, 2003 and entitled“MgO-Based Tunnel Spin Injectors” (now abandoned), both of which arehereby incorporated by reference.

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of contractMDA972-01-C-0051 awarded by DARPA.

TECHNICAL FIELD

This invention relates to tunnel junction injectors and detectors ofspin-polarized electrons for use in semiconductor spintronic devices.More particularly, this invention relates to a tunnel injector with highspin polarization formed from a ferromagnetic layer in conjunction witha MgO tunnel barrier on a semiconductor.

BACKGROUND OF THE INVENTION

The development of modern semiconductor electronics has followed Moore'slaw [G. E. Moore, Electronics 38, 114 (1965)] for several decades, withthe integration density doubling approximately every two years to giverise to ever more powerful and yet cheaper logic and memory devices.However, as the device minimum feature size shrinks towards fundamentalphysical limits, which will eventually preclude or slow down scaling toeven smaller sizes, there are increasing efforts to search foralternatives to conventional electronic devices. In particular,conventional microelectronic devices use the electron's charge or theflow of electron charge to build useful memory and logic devices. Theelectron has another fundamental property, its spin, which is a quantummechanical property that gives rise to magnetism. The spin of anelectron has two flavors and is characterized as either being up ordown. Electron spin-based microelectronics, or what is often referred toas spintronics, is emerging as a promising technology to rival orreplace charge-based electronics. In contrast to conventionalelectronics, where the electron charge in semiconducting materials isused for device operations, spintronics attempts to harness theelectron's spin to process and store information. It is widely believedthat spintronics has the potential to bring about a new generation ofelectronic devices with high speed and density, non-volatility and lowpower consumption [S. A. Wolf et al., Science 294, 1488 (2001)].

Spintronics takes advantage of the long relaxation time (>100 ns) [J. M.Kikkawa and D. D. Awschalom, Physical Review Letters 80, 4313 (1998)]and large coherence length (>100 μm) [J. M. Kikkawa and D. D. Awschalom,Nature 397, 139 (1999)] of electron spins within semiconductingmaterials. However, a major obstacle for semiconductor spintronics isthe electrical generation of highly spin-polarized carriers withinsemiconductors, which is often referred to as spin injection. Oncespin-polarized electrons or holes (vacancies of electrons in the valenceband) are injected they can then be subjected to further spinmanipulation and spin detection to create devices with newfunctionality. In III-V bulk semiconductors such as GaAs, the hole spinrelaxation is much faster than the electron spin relaxation due to thestrong spin-orbit interaction within the valence sub-bands.Consequently, the hole spin relaxation time is on the scale of themomentum relaxation time (˜100 fs), whereas the electron spin relaxationtime can be much longer than the momentum relaxation time. In GaAs basedquantum well structures, however, the splitting of the valence sub-bandsresults in a significant enhancement of the hole spin relaxation time.Hole relaxation times as long as 1 ns have been observed by Roussignolet al. in an n-modulation doped 7.5 nm thick GaAs/Al_(x)Ga_(1-x)Asquantum well [Physical Review B 46, 7292 (1992)]. It is anticipated thatelectron spin relaxation times should be even longer in doped siliconsemiconducting materials because of weaker spin-orbit coupling.

Various methods have been adopted to inject electron and hole spins intosemiconductors. The very first approaches simply attached ferromagneticmetal contacts to the surfaces of semiconductors and passed electricalcurrent from the metal contact into the semiconductor. Since it is wellknown that electrical current in ferromagnetic metals is usuallydominated by either the spin-up or spin-down electrons, it was supposedthat one could use such contacts to directly inject spin-polarizedcurrent into semiconductors. However, despite considerable effort overmany years the efficiency of spin injection from ferromagnetic metalsinto semiconductors through diffusive contacts has been determinedexperimentally to be very low [see, for example, Filip et al., PhysicalReview B 62, 9996 (2000)]. While for a long time this was regarded as aproblem of spin relaxation within the ferromagnet/semiconductor contactregion, perhaps due to the poor structural integrity of such contacts,it is now believed that the injection efficiency is fundamentallylimited by the mismatch in conductivity between typical ferromagneticmetals and semiconductors [Schmidt et al., Physical Review B 62, R4790(2000)].

One potential way around the conductivity-mismatch problem is to useferromagnetic contacts with lower electrical conductivities, such asmagnetic semiconductors. In 1999, two groups demonstrated spin injectionfrom two different dilute magnetic semiconducting materials into GaAsbased semiconducting heterostructures. Both groups used GaAs-basedquantum well (QW) light emitting diode (LED) structures to measure thespin polarization of the injected electrical current. Injected electrons(holes) are combined with holes (electrons) within the QW LED to emitphotons. The circular polarization of emitted light is indicative of thespin-polarization of the injected electrons or holes. Fiederling et al.[Nature 402, 787 (1999)] used a Mn and Be doped ZnSe alloy, BeMnZnSe, asa spin-injector for n-doped AlGaAs. BeMnZnSe is paramagnetic but has avery large g-factor, so that by applying large magnetic fields (severalTesla) the electronic levels are Zeeman split such that the lowestenergy conduction band states become spin-polarized. Fiederling et al.showed a significant degree of spin polarization of the injectedelectrons but only at very low temperatures (well below 30K) and inlarge magnetic fields. Ohno et al. [Nature 402, 790 (1999)] used Mndoped GaAs (GaMnAs) for spin injection into undoped GaAs but only foundevidence for very low spin polarization of the injected holes. GaMnAs isbelieved to be ferromagnetic for low concentrations of Mn dopants butonly at low temperatures. The Curie temperature of GaMnAs is below˜150K. Thus, neither of these dilute magnetic semiconductor spininjectors is useful for practical devices since they only operate at lowtemperatures.

Theoretical work by Rashba [Physical Review B 62, R16267 (2000)]proposed that the presence of a tunnel barrier between ferromagneticmetals and semiconductors could overcome the conductivity mismatchproblem, potentially allowing ferromagnetic metals to be used as spininjectors. Ferromagnetic metals are known to have Curie temperaturesmuch higher than room temperature, making them useful for deviceapplications. Hanbicki et al. [Applied Physics Letters 82, 4092 (2003)]utilized an Fe/GaAs Schottky tunnel barrier to realize spin injection. ASchottky tunnel barrier is typically formed when a metal is placed on asemiconductor, and is due to electronic energy band-bending in thesemiconductor and the formation of a depletion region in the surfaceregion of the semiconductor. The extent of the depletion region islargely governed by the concentration of charge carriers in this region,which itself is determined by the dopant concentration of thesemiconductor. As shown in FIG. 1A, Hanbicki et al. deposited a 10 nmthick Fe layer 32 on an AlGaAs/GaAs quantum well LED structure 11 bymolecular beam epitaxy (MBE). Electrons were injected from the Fe layer32 into the quantum well LED 11 by applying a bias voltage 42 across theentire structure in a perpendicular magnetic field oriented in thedirection given by arrow 52, which aligned the magnetic moment in theferromagnetic Fe layer 32 to be perpendicular to the film plane (i.e.,the plane defined by the interface of the LED structure 11 and the Felayer 32). The injected electrons recombined with holes in the quantumwell LED 11 and emitted light 62. According to the optical selectionrules [see, for example, “Optical Orientation”, NorthHolland, Amsterdam,1984, edited by Meier and Zakharchenya], the circular polarization ofthe light 62 in this geometry is correlated with the spin-polarizationof the injected electrons and, therefore, can be used to determine thespin injection efficiency. Hanbicki et al. measured a spin polarizationof 32% at 4.5 K using this optical detection technique. The measuredspin polarization, however, decreased rapidly at higher temperatures.Furthermore, to grow the Fe layer 32 with MBE is impractical for devicefabrication. The direct contact of the Fe layer 32 with thesemiconductor LED 11 could cause intermixing between the two and thuscompromise the device thermal stability.

As shown in FIG. 1B, Manago and Akinaga [Applied Physics Letters 81, 694(2002)] used a 2 nm thick Al₂O₃ tunnel barrier 24′ grown on anAlGaAs/GaAs quantum well LED 11′ for spin injection. A ferromagneticelectrode 32′, consisting of Co, Fe or Ni₈₀Fe₂₀, was deposited on top ofthe Al₂O₃ tunnel barrier 24′ and capped with a Au layer 34′. Electronswere injected from the ferromagnetic layer 32′ into the quantum well LED11′ by applying a bias voltage 42′ across the entire structure in aperpendicular magnetic field whose orientation is given by arrow 52′,with this magnetic field aligning the magnetic moment in theferromagnetic layer 32′ to be perpendicular to the film plane. Theinjected electrons recombined with holes in the quantum well LED 11′ andemitted light 62′, whose circular polarization was used to analyze theinjected electron spin polarization. Manago and Akinaga only observed asmall polarization of ˜1% at room temperature. As shown in FIG. 1C,Motsnyi et al. [Applied Physics Letters 81, 265 (2002)] formed an Al₂O₃tunnel barrier 24″ by oxidizing a 1.4 nm thick Al layer grown on anAlGaAs/GaAs layered structure 12″ for spin injection. A ferromagneticelectrode 32″, consisting of 2 nm Co₉₀Fe₁₀ followed by 8 nm Ni₈₀Fe₂₀,was deposited on top of the Al₂O₃ tunnel barrier 24″ and capped with 5nm Cu 34″. Electrons were injected from the ferromagnetic layer 32″ intothe semiconductor structure 12″ by applying a bias voltage 42″ acrossthe entire structure in an oblique magnetic field whose orientation isgiven by the arrow 52″. The injected electrons recombined with holes inthe semiconductor structure 12″ and emitted light 62″. In this geometry,the circular polarization of the light does not directly reflect theinjected electron spin polarization. Motsnyi et al. used the Hanleeffect to deduce the spin injection efficiency [“Optical Orientation”,NorthHolland, Amsterdam, 1984, edited by Meier and Zakharchenya]. Thedirectly measured light polarization was only a few percent at 80 K, andthe interpretation of the data could be further complicated by otherparasitic effects.

Thus none of these prior art spin injectors give significantspin-polarized electrons at room temperature. What is needed forsemiconductor spintronic devices is a source of highly spin-polarizedelectrons operating at room temperature and prepared using practicalfabrication techniques.

SUMMARY OF THE INVENTION

The present invention provides a MgO based tunnel spin injector as asource of highly spin-polarized electrons at useful temperatures forsemiconductor spintronic applications. The spin injector includes aferromagnetic metal electrode and a MgO tunnel barrier grown on asemiconductor. By applying a bias voltage across the entire structure,spin-polarized electrons can be transported into the semiconductor forfurther spin manipulation and detection. The spin injector of thepresent invention has many advantages over prior art spin injectorsincluding the high spin polarization of the injected electrons at roomtemperature and good thermal stability. A lower bound for spin injectionefficiency of ˜50% is observed at 100 K, which is expected to remainhigh up to room temperature due to the high Curie temperatures of theferromagnetic metals and the weak temperature dependence of thetunneling process.

The present invention also provides a method of forming a MgO basedtunnel spin injector. The MgO tunnel barrier is grown on a semiconductorby first depositing a thin metallic Mg layer followed by a MgO layerfabricated by depositing Mg in the presence of reactive oxygen.

The present invention also provides a method of improving the spininjection efficiency by thermal annealing. The thermal annealing, inaddition, provides evidence that good thermal stability is achieved inthe present invention.

The ferromagnetic layers disclosed herein can be formed from anyferromagnetic or ferrimagnetic metal or indeed any ferromagnetic orferrimagnetic material which is sufficiently conducting. In particular,these layers can be formed from ferrimagnetic metals such as Fe₃O₄, orfrom metallic ferromagnetic oxides such as oxides from the perovskitefamily, including the family of ferromagnetic manganites such asLa_(1-x)Sr_(x)MnO₃. Likewise, these layers can also be formed fromvarious half-metallic ferromagnetic metals including CrO₂, thehalf-Heusler alloys such as NiMnSb and PtMnSb and other ferromagneticHeusler and half-Heusler alloys.

One embodiment of the invention is a device that comprises a first layerthat includes at least one magnetic material from the group consistingof ferromagnetic materials and ferrimagnetic materials. The device alsocomprises a MgO tunnel barrier on and in contact with the first layer,and a second layer that includes semiconductor material. The secondlayer is on and in contact with the MgO tunnel barrier, so that the MgOtunnel barrier is sandwiched between the first layer and the secondlayer, thereby forming a first spintronic element. The first layer, theMgO tunnel barrier, and the second layer are configured to enablespin-polarized charge carrier transport between the semiconductor andthe magnetic material, so that spin-polarized charge carriers can flowinto and/or out of the semiconductor (e.g., electrons may be injectedinto the semiconductor). In one embodiment, at least one of the firstlayer and the second layer includes a spacer layer that is in contactwith the MgO tunnel barrier, in which the spacer layer does notsubstantially interfere with the tunneling properties of the MgO tunnelbarrier, thereby allowing charge carriers to substantially maintaintheir spin polarization as they pass through the spintronic element.Alternatively, the MgO tunnel barrier may be in direct contact with boththe semiconductor and the magnetic material. The first layer may includea ferromagnetic material, such as Fe, or an alloy of Co and Fe, in whichthe Fe concentration is between 8 and 50 atomic %, or more preferablybetween 12 and 25 atomic %. Alternatively, the ferromagnetic materialmay be an alloy of at least 2 metals selected from the group consistingof Ni, Fe, and Co, and the ferromagnetic material may advantageously bebcc and substantially (100) oriented. The device may further comprise alayer of antiferromagnetic material, with the ferromagnetic materialbeing exchange biased by the antiferromagnetic material, and thisantiferromagnetic material may include an alloy selected from the groupconsisting of Ir—Mn and Pt—Mn. In one embodiment, the layer offerromagnetic material may have a shape that is generally longer in onedirection than in another direction, thereby fixing the magnetic momentof the ferromagnetic material through shape magnetic anisotropy. Thedevice may further comprise a first lead that is in electricalcommunication with the semiconductor material, as well as a second leadthat is in electrical communication with the ferromagnetic material,with these leads providing voltage across the spintronic element toenable the flowing of spin-polarized charge carriers into and/or out ofthe spintronic element. The device may include antiferromagneticmaterial that is in electrical communication with both the ferromagneticmaterial and the second lead. The semiconductor may advantageously beGaAs; alternatively, the semiconductor may be selected from the groupconsisting of Al_(x)Ga_(1-x)As, In_(y)Ga_(1-y)As, ZnSe, GaN, InGaN,GaNInAs, GaSb, InGaSb, InP, InGaP, Si, Ge, SiGe, and heterostructuresthereof, in which x and y are between 0 and 100%. The MgO tunnel barriermay be substantially (100) oriented; preferably both the MgO tunnelbarrier and the ferromagnetic material are substantially (100) oriented,and the ferromagnetic material is bcc. The MgO tunnel barrier mayadvantageously have a thickness between 3 and 50 angstroms. In apreferred embodiment, the first layer, the MgO tunnel barrier, and thesecond layer are configured so that, upon application of a voltageacross the device, the spin polarization of current between the MgOtunnel barrier and the semiconductor is greater than 20%, or morepreferably greater than 40%. The device may further include a secondspintronic element that is in electronic communication with the firstspintronic element, with the first and the second spintronic elementstogether forming respective devices for spin injection and spindetection.

One aspect of the invention is a method of using the aforementionedfirst spintronic element. The method includes flowing charge carriersbetween a surface of the semiconductor and the magnetic material, acrossthe MgO tunnel barrier, in which the charge carriers undergo spindependent tunneling through the MgO tunnel barrier. The method furtherincludes detecting the spin polarization of the charge carriers. Thecharge carriers may include electrons or holes. The magnetic materialmay advantageously include a ferromagnetic material, and thesemiconductor material may include GaAs.

Another aspect of the invention is a method of using the aforementionedfirst spintronic element, in which a voltage is applied across thedevice, so that a potential difference is established between themagnetic material and the semiconductor material, thereby inducing theflow of spin-polarized charge carriers between the magnetic material andthe semiconductor material. The method may further include applying anelectromagnetic field to change the direction of the spin of the chargecarriers. The charge carriers may include electrons or holes. Themagnetic material may advantageously include a ferromagnetic material,and the semiconductor material may include GaAs.

Yet another aspect of the invention is a method that includes forming aMgO tunnel barrier on a surface of an underlayer (in which the surfaceis selected to be substantially free of oxide), and forming an overlayeron the MgO tunnel barrier to construct a spintronic element, in whichone of the underlayer and the overlayer includes a layer ofsemiconductor material, and the other of the underlayer and theoverlayer includes a layer of magnetic material selected from the groupconsisting of ferromagnetic materials and ferrimagnetic materials; inthe resulting spintronic element, a MgO tunnel barrier is sandwichedbetween the underlayer and the overlayer. In one preferredimplementation, the MgO tunnel barrier is in direct contact with boththe semiconductor material and the magnetic material. In anotherimplementation, at least one of the underlayer and the overlayerincludes a spacer layer that is in contact with the MgO tunnel barrier,in which the spacer layer is selected to not substantially interferewith the tunneling properties of the MgO tunnel barrier, therebyallowing charge carriers to substantially maintain their spinpolarization as they pass through the spintronic element. The magneticmaterial may include a ferromagnetic material, and the spintronicelement may further include a layer of antiferromagnetic material, inwhich the ferromagnetic material is exchange biased by theantiferromagnetic material. The method may further include annealing thespintronic element to increase the spin polarization of charge carrierspassed through the element. The charge carriers may be electrons orholes. The MgO tunnel barrier may advantageously be substantially (100)oriented. Also, the magnetic material may include ferromagneticmaterial, with the ferromagnetic material being bcc and substantially(100) oriented.

A preferred method of forming the MgO tunnel barrier on the underlayerincludes depositing Mg onto a surface of the underlayer to form a Mglayer thereon (in which the surface is selected to be substantially freeof oxide). The preferred method further includes directing additionalMg, in the presence of oxygen, towards the Mg layer to form a MgO tunnelbarrier in contact with the underlayer, in which the oxygen reacts withthe additional Mg and the Mg layer. The thickness of the Mg layer ispreferably selected to be large enough to prevent oxidation of theunderlayer but small enough that, upon reaction of the oxygen with theMg layer, substantially all the Mg in the Mg layer is converted intoMgO; the Mg layer, however, is preferably deposited in the absence ofsubstantial amounts of reactive oxygen. The resulting MgO tunnel barrieradvantageously has a thickness of between 3 and 50 angstroms. The methodpreferably further includes annealing MgO tunnel barrier to improve itstunneling characteristics.

One embodiment of the invention is a device that comprises a first layerthat includes at least one magnetic material from the group consistingof ferromagnetic materials and ferrimagnetic materials, with the firstlayer having a surface that is substantially free of oxide formed fromthe first layer (e.g., native oxide). The device further comprises a MgOtunnel barrier on and in contact with the surface of the first layer.The device also comprises a second layer that includes semiconductormaterial, with the second layer having a surface that is on and incontact with the MgO tunnel barrier, so that the MgO tunnel barrier issandwiched between the first layer and the second layer. In a preferredembodiment, the surface of the overlayer is substantially free of oxideformed from the overlayer (e.g., native oxide). In another embodiment,at least one of the overlayer and the underlayer includes a spacer layerthat is in contact with the MgO tunnel barrier, in which the spacerlayer is selected to not substantially interfere with the tunnelingproperties of the MgO tunnel barrier. The MgO tunnel barrier preferablyhas a thickness of between 3 and 50 angstroms. In a preferredembodiment, i) the amount of any oxide separating the MgO tunnel barrierfrom the overlayer and the underlayer is sufficiently low, and ii) theMgO tunnel barrier, the underlayer, and the overlayer are sufficientlyfree of defects, that the spin polarization of current flowing betweenthe MgO tunnel barrier and the semiconductor is greater than 20%, andmore preferably greater than 40%.

Yet another embodiment of the invention is a preferred spintronicelement that includes a magnetic layer of at least one ferromagneticand/or ferrimagnetic material, a MgO tunnel barrier, and asemiconducting layer, in which the MgO tunnel barrier is between themagnetic layer and the semiconducting layer. The magnetic layer, the MgOtunnel barrier, and the semiconducting layer are in proximity with eachother, so that, upon application of a voltage across the device, thespin polarization of current flowing between the MgO tunnel barrier andthe semiconducting layer is greater than 20% (or more preferably greaterthan 40%). The magnetic layer is preferably a ferromagnetic material(such as Fe), bcc, and substantially (100) oriented. The MgO tunnelbarrier is preferably also substantially (100) oriented and has athickness of between 3 and 50 angstroms. The spintronic element mayfurther include a layer of antiferromagnetic material, with theferromagnetic material being exchange biased by the antiferromagneticmaterial. The semiconducting layer may advantageously include GaAs, oranother semiconductor, such as Al_(x)Ga_(1-x)As, In_(y)Ga_(1-y)As, ZnSe,GaN, InGaN, GaNInAs, GaSb, InGaSb, InP, InGaP, Si, Ge, SiGe, andheterostructures thereof, in which x and y are between 0 and 100%. If asecond spintronic element in electronic communication with the firstspintronic element is used, the first and the second spintronic elementstogether may form respective devices for spin injection and spindetection.

A method of using the aforementioned preferred spintronic elementincludes flowing charge carriers between a surface of the semiconductinglayer and the magnetic material (in which the charge carriers undergospin dependent tunneling through the MgO tunnel barrier) and detectingthe spin polarization of the charge carriers. The charge carriers mayinclude electrons or holes. The magnetic material preferably includesferromagnetic material, and the semiconducting layer may advantageouslyinclude GaAs.

Another method of using the aforementioned preferred spintronic elementincludes applying a voltage across it, so that a potential difference isestablished between the magnetic material and the semiconducting layer,thereby inducing the flow of spin-polarized charge carriers between themagnetic material and the semiconducting layer. The method may furtherinclude applying an electromagnetic field to change the direction of thespin of the charge carriers, which may include electrons or holes. Themagnetic material preferably includes a ferromagnetic material, and thesemiconducting layer may advantageously include GaAs.

For several aspects and embodiments of the invention disclosed herein, aMgO tunnel barrier is sandwiched between an underlayer and an overlayer,one of which includes one or more layers of a ferromagnetic materialand/or a ferrimagnetic material, and the other of which includes asemiconductor. While the MgO tunnel barrier is preferably in directcontact with the ferromagnetic material, ferrimagnetic material and/orsemiconductor, each of the underlayer and overlayer may optionallyinclude one or more spacer layers which are adjacent to the MgO tunnelbarrier but which do not significantly affect the tunneling propertiesof the MgO layer, e.g., by not significantly diminishing the spinpolarization of electrons tunneling through the MgO tunnel barrier.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a cross section of a prior art spin injector that uses anFe/GaAs Schottky tunnel barrier injector, in which the electron spinpolarization is measured by optical detection of the polarization of thelight emitted from the semiconductor quantum well in a perpendicularmagnetic field.

FIG. 1B shows a cross section of a prior art spin injector that uses aferromagnetic metal/Al₂O₃ tunnel barrier injector, in which the electronspin polarization is measured by optical detection of the polarizationof the light emitted from the semiconductor quantum well in aperpendicular magnetic field.

FIG. 1C shows a cross section of a prior art spin injector that uses aferromagnetic metal/Al₂O₃ tunnel barrier injector, in which the electronspin polarization is measured using the Hanle effect in an obliquemagnetic field.

FIG. 2A illustrates the various layers that are deposited to form a spininjector device of the present invention, with the final structure beingshown in FIG. 2B.

FIG. 2B shows a cross section of a spin injector that uses aCo₇₀Fe₃₀/MgO tunnel barrier injector, in which the electron spinpolarization is measured by optical detection of the polarization of thelight emitted from the semiconductor quantum well in a perpendicularmagnetic field.

FIG. 3 shows electroluminescence spectra using a Co₇₀Fe₃₀/MgO tunnelspin injector at 100 K and with a bias voltage of 1.8 V, recorded forthree different magnetic fields: 0 and ±5 T.

FIG. 4A shows the electroluminescence polarization before backgroundsubtraction (open circles) as a function of magnetic field, using aCo₇₀Fe₃₀/MgO tunnel spin injector at 100 K and with a bias voltage of1.8 V.

FIG. 4B shows the electroluminescence polarization after backgroundsubtraction (open circles) as a function of magnetic field, using aCo₇₀Fe₃₀/MgO tunnel spin injector at 100 K and with a bias voltage of1.8 V, in which the perpendicular magnetic moment of the Co₇₀Fe₃₀ layer(solid line) was measured with a superconducting quantum interferencedevice magnetometer at 20 K.

FIG. 5 shows the electroluminescence polarization after backgroundsubtraction (solid circles) as a function of temperature, using aCo₇₀Fe₃₀/MgO tunnel spin injector with a bias voltage of 1.8 V.

FIG. 6 shows the electroluminescence polarization after backgroundsubtraction as a function of the bias voltage, using a Co₇₀Fe₃₀/MgOtunnel spin injector at 100 K, in which the solid and open circlesrepresent results for the sample before and after annealing at 300° C.for 60 minutes, respectively.

FIG. 7 shows a cross section of a spin injector including an exchangebiased ferromagnetic layer.

FIG. 8 shows a cross section of a spintronic device that utilizes both aspin injector and a spin detector.

FIG. 9 shows a cross section of a spin injector that includes spacerlayers adjacent to the MgO tunnel barrier.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Theoretical calculations have predicted that tunneling structures basedon crystalline MgO tunnel barriers can have much higher tunneling spinpolarization compared to conventional tunneling structures based onamorphous Al₂O₃ tunnel barriers [Butler et al., Physical Review B 63,054416 (2001); Mathon and Umerski, Physical Review B 63, 220403(R)(2001)]. However, prior art methods used to deposit MgO tunnel barriershave shown low tunneling spin polarization values most likely because offerromagnetic oxides formed at the interface between the MgO tunnelbarrier and the ferromagnetic layers. There have been several studies onthe epitaxial growth of MgO onto GaAs substrates as buffer layers forthe growth of superconducting or ferroelectric films. These prior artmethods of forming MgO layers on GaAs include pulsed laser deposition,electron beam evaporation and magnetron sputtering of MgO layersdirectly onto GaAs or passivated GaAs surfaces. For example Bruley etal. [Appl. Phys. Lett. 65, 564 (1996)] use magnetron sputtering todeposit MgO layers onto (001) oriented GaAs but find that the quality ofthe MgO/GaAs interface is very poor. Cross-sectional high resolutiontransmission electron microscopy reveals that there is a 10-200 Å thickinterface region between the MgO layer and the GaAs substrate which isamorphous and which contains both MgO as well as oxides of GaAs. Thusthese prior art methods of forming MgO on GaAs are not practical for therealization of the high quality MgO/GaAs interfaces needed forspintronic applications, since they lead to loss of spin polarization ofthe injected electrons.

FIG. 2A shows the various layers that are deposited to form an exemplaryspin injector device of the present invention. As discussed in moredetail below, a MgO-based tunnel spin injector 200 is formed as a resultof this process and is shown in FIG. 2B. The spin injector 200 is grownon a GaAs layer 115 which is crystallographically (100) oriented. TheGaAs layer is grown on top of an AlGaAs/GaAs quantum well LED structure111 which is itself grown on a p⁺-GaAs single crystalline (100) orientedsubstrate 105. The LED structure 111, which is not shown in detail, isincorporated here for the purposes of detecting and measuring themagnitude of the spin polarization of the injected current: The spininjector does not require the presence of the LED structure. The LEDheterostructure 111 was grown in an MBE chamber and consists of thefollowing layers grown on the p⁺-GaAs substrate 105: 200 nm Be dopedp-GaAs (1.1×10¹⁹ cm⁻³)/190 nm Be doped p-Al_(0.08)Ga_(0.92)As (1×10¹⁹cm⁻³)/280 nm Be doped p-Al_(0.08)Ga_(0.92)As (2×10¹⁸ cm⁻³)/100 nm Bedoped p-Al_(0.08)Ga_(0.92)As (1×10¹⁷ cm⁻³)/10 nm Be dopedp-Al_(0.08)Ga_(0.92)As (1×10¹⁵ cm⁻³)/75 nm undopedAl_(0.08)Ga_(0.92)As/10 nm undoped GaAs/15 nm undopedAl_(0.08)Ga_(0.92)As/100 nm Si doped n-Al_(0.08)Ga_(0.92)As (5×10¹⁶cm⁻³). The active region of the LED 111 is the 10 nm undoped GaAsquantum well sandwiched between the 75 nm undoped Al_(0.08)Ga_(0.92)Aslayer and the 15 nm undoped Al_(0.08)Ga_(0.92)As layer. The GaAs layer115 is comprised of 5 nm undoped GaAs. Finally, the GaAs layer 115 iscovered with a layer of arsenic (not shown) for the purpose ofprotecting this layer when it is exposed to air after the wafer isremoved from the MBE chamber. The wafer is then transferred to a sputterdeposition system for the purposes of forming the spin injector 200 bysputter deposition. The wafer is first heated at 550° C. for 15 minutesto remove the arsenic capping layer. After the sample cools down to roomtemperature, the spin injector structure 200 is grown.

All the layers of the spin injector were deposited at nominally roomtemperature, although the ambient temperature at the substrate positionwithin the sputter deposition system is typically a little higher thanroom temperature because of the considerable energy injected into themagnetron plasma sources. Thus, the temperature at the substrates islikely in the range of 40 to 50° C.

The spin injector 200 is essentially comprised of a MgO tunnel barrierand a ferromagnetic layer, which are formed on top of the GaAs layer115. Since the surface of the GaAs layer 115 is readily oxidized, theMgO tunnel barrier is formed by a method that prevents the oxidation ofthe GaAs surface layer but yet forms a high quality MgO tunnel barrier.(The other layers and components referred to herein may be constructedusing techniques known to those skilled in the art.) For this purposethe MgO tunnel barrier is formed by first depositing a thin metallic Mglayer 122 (FIG. 2A) in the absence of any oxygen. The Mg layer 122 mustbe sufficiently thick to completely cover the GaAs layer 115 so as toprevent oxidation of the GaAs surface. The Mg layer preferably has athickness of between 3 and 20 angstroms, more preferably between 3 and10 angstroms, and still more preferably between 4 and 8 angstroms.

For the structure described here, a 1 nm thick Mg layer 122 (FIG. 2A)was deposited by dc magnetron sputtering in 3 mTorr argon on top of theGaAs layer 115. The MgO barrier is then formed by depositing a 3.1 nmthick MgO layer 124 (FIG. 2A) on the Mg layer 122 by reactive sputteringfrom a metallic Mg target in a gas mixture of 97% argon and 3% oxygen at3 mTorr gas pressure. The ratio of argon and oxygen during the MgOdeposition has been found to be important in obtaining a high-qualityMgO barrier, although the optimized ratio depends on the particularsputtering system used for the deposition of the MgO layer 124. Thequality of the MgO tunnel barrier can be judged in several ways. Perhapsthe most straightforward is from the performance of the spin injectordevice 200 as determined from the magnitude of the spin polarization ofthe injected electrons (or holes). The oxygen content of the sputteringgas used during the deposition of the MgO layer can be varied so as tooptimize the spin polarization of the tunneling current.

Although the MgO tunnel barrier is formed as a bilayer by firstdepositing a Mg layer 122 and then depositing a MgO layer 124 bydepositing Mg in the presence of reactive oxygen (as shown in FIG. 2A),the Mg layer 122 is substantially all oxidized during the process offorming the layer 124, so that the bilayer behaves and structurallylooks like a single substantially homogeneous layer 123 of MgO (asillustrated in FIG. 2B). The conditions of formation of the layer 124are chosen so that this layer 124 is formed in the presence of asufficient quantity of sufficiently reactive oxygen that the Mg layer122 is oxidized during the deposition of layer 124. Although it ispossible that the layer 122 is not completely oxidized, analysis bycross-section transmission electron microscopy does not readilydistinguish, either by composition or texture, the two layers 122 and124, which rather become a single layer 123 of MgO.

By using a (100) oriented GaAs substrate, the MgO layer 123, which isformed on top of the GaAs layer 115, is also textured in the (100)direction. In magnetic tunnel junction devices the largest tunnelingmagnetoresistance values are found for tunnel junction devices in whichthe MgO tunnel barrier is textured in the (100) orientation and theferromagnetic layers adjacent to each side of the MgO tunnel barrierhave a bcc crystal structure and are oriented in the (100) crystalorientation. Thus, to obtain the highest spin polarization of currentfrom the spin injector device 200, the MgO tunnel barrier is preferablyoriented in the (100) direction. The crystal structures of MgO and GaAsare well lattice matched, so that the crystal orientation of the MgOtunnel barrier follows that of the GaAs underlayer, so that the MgOtunnel barrier 123 is (100) oriented when it is deposited on a (100)oriented GaAs substrate 115.

The MgO layer 124 is formed by reactive sputtering from a metallic Mgtarget in an argon-oxygen gas mixture. The sputtering gas ispredominantly formed from argon, with oxygen being in the range of 1.5to 9% by volumetric flow of gas at standard temperature and pressure.Thus this corresponds to the relative molecular percentage of thecomponents in the gas mixture. The oxygen partial pressure is keptsufficiently low to prevent “poisoning” of the metallic Mg sputteringtarget but is sufficient to form a fully oxidized MgO layer. Poisoningof the target can lead to irreproducible deposition, especially when thesame target is used to form both the Mg layer 122 and the MgO layer 124.Conditioning of the target between depositing the Mg and MgO layers isvery helpful in obtaining reproducible results. Conditioning may becarried out by pre-sputtering the target, either in argon prior todeposition of the Mg layer 122 or in argon-oxygen prior to deposition ofthe MgO layer 124. The optimum concentration of oxygen in the sputteringgas depends on the detailed geometry and size of the deposition chamber,the pumping speed of the vacuum pumps attached to the system, as well asthe power applied to the Mg sputtering source. The deposition rate ofthe MgO will be influenced by the power applied to the Mg sputter gun,the oxygen concentration in the sputter gas, and the sputtering gaspressure, as well as geometric factors such as the distance from thesputter source to the substrate. Typically, it is preferred to use thesmallest possible amount of oxygen not only to prevent contamination ofthe deposition chamber and other sputtering targets in the chamber, butalso because the thermal stability of the MgO tunnel barrier isinfluenced by small amounts of excess oxygen in or on top of this layer.

The MgO layer 124 may be formed by various deposition methods inaddition to reactive sputter deposition. Any method which delivers bothMg and oxygen in a sufficiently reactive state to form MgO during thedeposition of the Mg and oxygen is suitable. For example, the Mg can bedeposited by ion beam sputtering from a Mg target in the presence ofoxygen generated from a source of atomic oxygen such as an rf ormicrowave source. Similarly, the MgO can be deposited by ion beamsputtering from a Mg target in the presence of reactive oxygen deliveredfrom an ion-assist source. The MgO layer 124 can also be evaporated froma source of MgO, for example, by electron beam evaporation using a beamneutralizer, or by evaporation from a crucible or from a Knudsen source.The MgO layer 124 can also be formed by deposition from a MgO source inthe presence of atomic oxygen provided by an rf or microwave source orany other source of sufficiently reactive oxygen. The MgO layer can alsobe formed by pulsed laser deposition either by using a MgO target or aMg target in the presence of sufficiently reactive oxygen. The MgO layermay also be formed by reactive sputtering from a Mg target using varioussputtering gas mixtures, provided that oxygen is present. For example,Argon can be replaced by other rare gases, for example, Neon or Kryptonor Xenon. The Mg in the underlayer 122 and the MgO layer 124 are ideallyfree of impurities; the Mg preferably contains less than 5 atomic % ofimpurities, and more preferably less than 1 atomic % of impurities, soas to not substantially affect the tunneling properties of the MgOtunnel barrier, which would, for example affect the spin polarization ofthe injected current in a spin injector device.

For the purposes of the test structure described here, both the Mg layer122 and the MgO layer 124 were deposited through a rectangularly (1×8mm²) shaped shadow mask. Next, a 50 nm thick Al₂O₃ isolation layer isgrown in a gas mixture of 93% argon and 7% oxygen at 3 mTorr gaspressure through a second metal shadow mask. This layer covers regionsof the MgO layer 123 to form pairs of circular isolation pads 172 withgaps between these pads ˜300 μm wide. The pads 172 have diameters of˜2.5 mm. Finally, a ferromagnetic layer 132 formed from a 5 nm thickCo₇₀Fe₃₀ layer is deposited through a third shadow mask in the shape ofa dogbone. This layer 132 is largely deposited on top of each pair ofpads 172 formed from the isolation layer but also covers the small areaof exposed MgO layer 123 between these pads. Thus this sequence ofshadow masks creates small regions of ferromagnetic material 132approximately 100×300 μm² in size directly on top of the MgO tunnelbarrier 123, which defines the active area of the tunnel injector. Theisolation layer pads 172 are used to electrically isolate theferromagnetic electrode 132 from the GaAs layer 115. The ferromagneticelectrode 132 is capped with a 10 nm thick Ta layer 134 to preventoxidation. This capping layer 134 is deposited through the same shadowmask used to define the ferromagnetic layer 132. Both metal layers 132and 134 are sputtered in 3 mTorr argon.

The use of shadow masks means that the edges of the MgO layer will beexposed to air. The humidity of the air may deleteriously affect the MgOlayer, since MgO is hygroscopic and easily degrades in the presence ofwater to form a hydroxide which is much less stable. In actual devicesthe spin injector and the MgO are likely encapsulated by an oxide orother protective material.

The choice of the capping layer material may be determined by the effectof these structures on the magnetic properties of the underlying layer132, or by the required thermal stability of the device; in this latterregard, TaN may be preferred for the capping layer 134. The layer 134may also be comprised of various other metals, such as Cu, W, WTi, andTiN. The layer 132 may include one or more ferromagnetic orferrimagnetic materials selected to give highly spin-polarized currentthrough the MgO tunnel barrier 123.

The preferred thickness of the Mg layer 122 is in the range of 3 to 20Å. The minimum thickness of the Mg layer 122 is determined by the amountof Mg required to completely cover the GaAs surface layer 115. This willdepend, for example, on the roughness of the surface of the GaAs layer115, the temperature at which the Mg layer 122 is deposited, and theenergy of the deposited Mg atoms. Typically, the layer 122 will besmoother the lower the deposition temperature, so that nominally roomtemperature deposition is preferred. Lower temperature deposition maygive rise to smoother layers but is less practical for manufacturing.The thickness of the MgO layer 122 can vary from ultra thin layers justa few angstroms thick to layers several tens of angstroms in thickness,but the preferred thickness will be determined by the devicerequirements for the resistance of the tunnel injector. The resistanceof the injector increases inversely with the area of the active area ofthe tunnel injector, and the resistance also increases approximatelyexponentially with the thickness of the MgO layer 123. Thus for mostdevice applications, the preferred thickness of the MgO layer 124 is inthe range from 1 to 10 angstroms. The thickness of the resulting MgOlayer 123 is preferably in the range of 3-50 angstroms, more preferably3-30 angstroms, still more preferably 3-20 angstroms, and mostpreferably 4-15 angstroms.

Electrons are injected from the spin injector 200 into the semiconductorstructure 115 by applying a bias voltage 142 across the entire structurein a perpendicular magnetic field whose orientation is given by thearrow 152. This magnetic field aligns the magnetic moment of theCo₇₀Fe₃₀ layer 132 to be perpendicular to the film plane, as indicatedby the arrow 153A in FIG. 2B. The plane of the film is the planedefined, for example, by the interface between the MgO layer 123 and theCoFe layer 132. A perpendicular magnetic field 152 is required only forthe purposes of optically detecting and measuring the spin polarizationof the injected electrons with the QW LED detector 111. To operate thespin injector for the purpose of injecting a spin-polarized current intothe GaAs layer 115, no magnetic field is necessarily required. Amagnetic field can be applied to the spin injector 200 in order tomagnetize the ferromagnetic layer 132 in a given direction either in theplane of the film (indicated by the arrow 153B in FIG. 7), orperpendicular to the film plane (indicated by the arrow 153A in FIG.2B), or at some angle to the film plane. The spin polarization of theinjected electrons will be aligned along the direction of themagnetization of the ferromagnetic layer 132, so by changing thedirection of the magnetic moment of layer 132, the direction of the spinof the injected electrons can also be varied. The spin injector can beincorporated into a more complex microelectronic device in which aprovision for magnetizing the layer 132 in different directions can bemade. For example, this can be achieved by passing current or currentpulses through micro-fabricated metal wires close to the spin injector200. The direction of magnetization of the layer 132 can either be setin a given direction for the duration of the time during which thecurrent is injected, or the remnant state of the magnetization of theferromagnetic layer 132 can be changed, so that after the remnant stateis set in a given direction, spin-polarized current can be injected fromthis ferromagnetic electrode.

The injected electrons recombine with holes in the quantum well LED 111and emit light 162. The circular polarization of thiselectroluminescence (EL) signal is analyzed by a combination of a liquidcrystal retarder and a linear polarizer to give the intensities of theleft (σ+) and right (σ−) circular polarization components. The spectrumof the selected circular polarization component is recorded with agrating spectrometer and a charge-coupled device (not shown).

Shown in FIG. 3 is the EL spectrum of the device shown in FIG. 2B, at atemperature of 100 K and with a bias voltage of 1.8 V, for threedifferent magnetic fields H=0 and ±5 T. The thick and thin linesrepresent the spectra of the σ+ and σ− circular polarization componentsof the EL, respectively. There are two types of holes in thesemiconductor quantum well: heavy holes (HH) and light holes (LH). TheEL peaks at the longer and shorter wavelengths are due to electronrecombination with the HH and LH, respectively. According to the opticalselection rules, the circular polarization of HH emission is equal tothe electron spin polarization just before the electrons recombine withthe HH [“Optical Orientation”, NorthHolland, Amsterdam, 1984, edited byMeier and Zakharchenya]. The heavy hole EL intensities of the σ+ (I⁺)and σ− (I⁻) circular polarization components are coincident in theabsence of a magnetic field, because the magnetic moment in the Co₇₀Fe₃₀layer 132 is aligned parallel to the plane of the QW structure 111,whereas the light 162 is emitted in a direction perpendicular to thisplane. The plane of the QW device is parallel to the plane formed by theCoFe layer 132 and the MgO layer 123. Applying a large field H=±5 T(whose orientation is given by the arrow 152) brings the moment of theCo₇₀Fe₃₀ layer 132 parallel to the direction of propagation of theemitted light 162 (as shown in FIG. 2B), so that significant heavy holeEL polarization is now observed. The sign of the light polarizationdepends on the direction of the magnetic moment of the Co₇₀Fe₃₀ layer132 and so changes when the magnetic field direction is reversed. Theheavy hole EL polarization P_(EL), defined as P_(EL)=(I⁺−I⁻)/(I⁺+I⁻),reaches ˜50% at 5 T.

FIG. 4A shows P_(EL) as a function of the magnetic field H appliedperpendicular to the plane of the ferromagnetic electrode 132. P_(EL)increases rapidly with increasing H as the magnetic moment of theCo₇₀Fe₃₀ layer 132 is rotated out of plane by the magnetic field 152. Atfields above ˜2 T, when the Co₇₀Fe₃₀ magnetic moment is rotatedcompletely out of the plane, P_(EL) continues to increase with H but ata much slower rate. P_(EL) increases approximately linearly at a rate of˜1.7%/T. A polarization of ˜50% is obtained at 5 T. The linear increaseof P_(EL) with the magnetic field (referred to as the linear“background” polarization, hereafter) is due to the suppression of spinrelaxation by the perpendicular magnetic field. Electrons injected intothe semiconductor 115 can lose their initial spin polarization throughspin relaxation processes before they recombine with the holes. One ofthe most important spin relaxation mechanisms in GaAs-basedsemiconductor heterostructures is the D'yakonov-Perel' (DP) mechanism,which is due to spin precession about an effective magnetic field whosemagnitude and direction depend on the electron momentum [D'yakonov andKachorovskii, Soviet Physics-Semiconductors 20, 110 (1986)]. It is wellknown that a perpendicular magnetic field can suppress the DP spinrelaxation [“Optical Orientation”, NorthHolland, Amsterdam, 1984, editedby Meier and Zakharchenya]. Therefore, the spin relaxation is reduced atlarger perpendicular magnetic fields and a higher spin polarization ismeasured.

FIG. 4B shows the dependence of spin polarization P_(C) on perpendicularmagnetic field (see FIG. 2B) after subtraction of the linear backgroundpolarization, such that P_(C)=P_(EL)−1.7%/T×H. P_(C) is shown as opencircles in FIG. 4B. A polarization of ˜42% is obtained for magneticfields larger than ˜2 T. The solid line in FIG. 4B corresponds to thecomponent of the magnetization of the Co₇₀Fe₃₀ layer 132 measuredperpendicularly to its plane at 20 K as a function of a perpendicularmagnetic field. These data were measured with a superconducting quantuminterference device (SQUID) magnetometer. Although P_(C) is measured at100K and the SQUID magnetization data are measured at 20K, there is avery weak temperature dependence of magnetization of the Co₇₀Fe₃₀ layer132 in this temperature regime. The excellent agreement between the ELdata and the SQUID data confirms that the spin polarization inside thesemiconductor structure 111 is indeed due to spin injection from theCo₇₀Fe₃₀ layer 132.

The Al_(0.08)Ga_(0.92)As/GaAs quantum well in the semiconductor LED 111has a very low light emitting efficiency for temperatures above 100 Kdue to the small confinement potential of the Al_(0.08)Ga_(0.92)Aslayers. Consequently, the spin injection experiments are limited by theoptical detection efficiency to temperatures no higher than ˜100 K.However, ferromagnetic metals normally have very high Curie temperatures(>1000 K) and thus can maintain their spin polarization at roomtemperature. Meanwhile, the tunneling process only has a weaktemperature dependence. Therefore, the very high spin injectionefficiency observed at 100 K will remain high up to room temperature.

Shown in FIG. 5 is P_(C) as a function of temperature at a bias voltageof 1.8 V for the device shown in FIG. 2B. A dramatic non-monotonictemperature dependence is observed, which is again most likelyassociated with the DP spin relaxation mechanism, as described in arecent theoretical paper by Puller et al. [Physical Review B 67, 155309(2003)]. DP spin relaxation is due to spin precession about an effectivemagnetic field whose magnitude and direction depends on the electronmomentum. Electron momentum scattering tends to randomize the effectivemagnetic field direction and thus reduce the averaging effect of thismagnetic field. Enhanced momentum scattering therefore suppresses DPspin relaxation. The momentum scattering rate in GaAs has a minimum fortemperatures in the range ˜40-60 K, as reported by Wolfe et al. inJournal of Applied Physics 41, 3088 (1970). Consequently, the DP spinrelaxation is more efficient at ˜60 K, giving rise to a minimum in thespin polarization at this temperature as observed in FIG. 5. Thetemperature dependence of P_(C) is thus a strong signature of DP spinrelaxation in the QW. Other spin relaxation mechanisms could not accountfor the observed temperature dependence. Model calculations based on theDP mechanism indicate that the observed temperature dependence of P_(C)can be well accounted for by DP spin relaxation within the quantum wellitself and that, therefore, the temperature dependence of the spininjection efficiency is actually quite weak over the temperature rangefrom 4 to 100 K. Indeed, there may well be some spin relaxation in theGaAs layers before the electrons enter the QW, so that P_(C) sets alower bound for the spin injection efficiency. In particular, over thetemperature range from 4 to 100 K these experiments indicate that thespin polarization of the injected electrons is more than 50% which isvery high.

The solid circles in FIG. 6 correspond to P_(C) measured as a functionof the bias voltage at 100 K. The decrease of P_(C) with the biasvoltage can be attributed to DP spin relaxation processes. The magnitudeof the effective magnetic field in the DP relaxation process depends onthe electron momentum. The larger electron momentum at higher biasesleads to a larger effective magnetic field, which results in moreefficient spin relaxation before the electrons recombine with the heavyholes. This consequently leads to a smaller detected spin polarization.

Higher voltages across the MgO tunnel barrier 123 lead to greatercurrent injection into the semiconductor 115. The current increasesnon-linearly with the voltage across the MgO barrier layer 123 becauseof the non-linear characteristics of the MgO tunnel injector and the QWLED 111. The maximum voltage which can be applied to the device islimited by the breakdown voltage of the MgO tunnel barrier 123. In FIG.6 the voltage 142 corresponds to the voltage drop across the MgO tunnelbarrier 123 plus the voltage drop across the QW LED detector 111. Thebreakdown voltage of the MgO tunnel barrier is about 1 volt per 1 nmthickness of the MgO layer 123.

Studies of the thermal stability of the spin injector 200 were carriedout. Thermal anneal treatments improve the magnitude of the measuredspin polarization of the spin injector device. Data comparing the biasvoltage dependence of P_(C) before (solid circles) and after an annealtreatment at 300° C. for 60 minutes in a high vacuum annealing furnace(open circles) are shown in FIG. 6. As shown in the figure, thepolarization of the electroluminescence, which is the spin polarizationof the injected electrons, is substantially increased after the annealtreatment at 300° C. For a bias voltage of 1.8 V the polarization isincreased from 42% to 52% at a measurement temperature of 100 K. Evenhigher polarizations can be realized after higher temperature anneals.After the anneal, the sample is cooled in vacuum in the annealingfurnace to ˜50° C. before being exposed to air. Thermal annealtreatments most likely improve the interface between the Co₇₀Fe₃₀ layer132 and the MgO barrier 123 and therefore result in a higher tunnelingspin polarization and thus a higher spin injection efficiency. Thermalanneal treatments may also improve the MgO tunnel barrier 123 by, forexample, improving the degree of oxidation of the Mg layer 122, perhapsthrough redistribution of oxygen from the layer 124 to the layer 122.Moreover, this result also indicates that the separation of theferromagnetic metal layer 132 and the semiconductor structure 115 by theMgO tunnel barrier 123 prevents intermixing of the two and gives rise tothermally stable structures. Thermal annealing may also improve thecrystallographic texture of the MgO layer 123 and the surroundinglayers, so that the structure of the MgO layer 123 prior to annealingmay not be substantially (100) oriented, but after thermal annealtreatments in the temperature range described above the crystallographictexture becomes substantially (100) oriented. The improvement of the MgOtunnel barrier on annealing can be monitored from the spin polarizationof the current injected into the semiconductor.

The methods of forming the MgO tunnel barriers described herein providetunnel barriers which are largely free of defects, which might otherwiseimpede the tunneling of charge carriers across the barrier. For example,defects in the barrier may provide electronic states in the barrier intowhich charge carriers may hop and reside for sufficient periods of timethat they lose their spin polarization or spin memory. Similarly, thesemethods provide an interface between the MgO tunnel barrier and theunderlayer which is largely free of oxide formed from the underlayer,which would otherwise degrade the performance of the spin injector, incontrast to prior art methods of forming MgO tunnel barriers.

Although the layers comprising the spin injector 200 were deposited atnominally room temperature, the preferred deposition temperatures willdepend on the detailed structure and composition of these layers.

Although the data shown in FIGS. 3 through 6 were obtained usingCo₇₀Fe₃₀ as the ferromagnetic electrode 132, other ferromagnetic metalscan also be used as the ferromagnetic electrode, such as Co, Fe, Ni andtheir binary and ternary alloys. In magnetic tunnel junctions (MTJs)with MgO tunnel barriers (formed using the methodology disclosedherein), very large tunneling magnetoresistance (TMR) values exceeding100% at room temperature are found for MTJs in which the ferromagneticelectrodes are formed from Fe or Co_(100-x)Fe_(x) alloys with the bcccrystal structure, and in which these alloys are textured with their(100) crystal axis perpendicular to the plane of the ferromagnetic andMgO layers; similarly, very large values of the spin polarization ofcurrent tunneling from Fe or Co_(100-x)Fe_(x) alloys through MgO tunnelbarriers are found using superconducting tunneling spectroscopy (STS).Thus, for the spin injector 200 of the current application, thepreferred ferromagnetic metals for the layer 132 which give the highestvalues of spin-polarized current are bcc alloys of Fe, Co—Fe and Ni—Feand, more particularly, Fe and Co_(100-x)Fe_(x) alloys with x in therange from 8 to 50 atomic percent, and the preferred crystallographicorientation of these layers is (100). The thermal stability of theMgO/CoFe interface is greatest for Co—Fe alloys with Fe content in therange of 12 to 25 atomic %. The thermal stability of this interface canbe as high as 400° C. depending particularly on the composition of theCo—Fe alloy and the amount of oxygen used in fabricating the MgO layer124. Using the preferred ferromagnetic metals, the spin polarization ofthe injected current using the spin injector of the current applicationcan exceed 76%, which is considerably higher than is possible usingprior art spin injectors based on ferromagnetic metals. Even when theferromagnetic layer is not formed from the preferred ferromagneticmetals, the spin injector of FIG. 2B nevertheless still shows high spinpolarization in the range of 50% spin-polarized current, which is alsomuch higher than has previously been obtained using prior artferromagnetic metal spin injectors.

Ferromagnetic half-metals such as Fe₃O₄, CrO₂, the Heusler and halfHeusler alloys such as NiMnSb and PtMnSb can also be used as theferromagnetic layer 132.

The ferromagnetic layer 132 can also be formed from more than oneferromagnetic layer in order to provide an improved spin injector. Forexample, it may be preferred to form the interface layer in directcontact with the MgO tunnel barrier 123 from an Fe layer or a Co—Fealloy in order to obtain the highest possible spin polarization. Thespin polarization is strongly dependent on the interface between the MgOlayer 123 and the ferromagnetic layer 132. However, in order to reducethe magnetostatic stray fields from the edges of layer 132, which becomelarger as the lateral dimensions of the layer 132 are decreased, it maybe preferred to reduce the magnetic moment of the layer 132, either byminimizing its thickness or preferably by forming the bulk of layer 132from a low magnetization magnetic material such as NiFe or a CoNiFealloy or by forming low magnetization alloys by alloying Ni, Co and Feand their binary and ternary alloys with non-magnetic elements.

It may also be preferred to form the ferromagnetic electrode 132 from asandwich of two antiferromagnetically coupled ferromagnetic layers forthe purposes of reducing the stray magnetostatic fields from the edgesof the ferromagnetic electrode. As described in U.S. Pat. No. 5,465,185to Parkin and Heim and U.S. Pat. No. 6,153,320 to Parkin, this can beaccomplished by forming the electrode 132 from a sandwich of two thinferromagnetic layers separated by a thin layer of Ru or Os or an alloyof Ru and Os.

The essential components of the spin injector 200 are the MgO tunnelbarrier 123 (formed by depositing an Mg layer 122 and the MgO layer 124)and the ferromagnetic electrode 132, as illustrated in FIG. 7 (as wellas FIG. 2B). The spin injector 200 and the semiconducting layer 115together form a spintronic element 204. It may be preferred to fix thedirection of the magnetic moment of layer 132 in a particular direction,such as is illustrated by the direction of the arrow 153B in FIG. 7.This direction can be in the plane of the ferromagnetic layer 132 asindicated by the arrow 153B, or it can be perpendicular to the plane asindicated by the arrow 153A in FIG. 2B. The magnetic moment of theferromagnetic layer 132 can be fixed in a particular direction by usingmagnetic shape anisotropy by forming the ferromagnetic electrode in ashape that is generally longer in one direction than in anotherdirection. The magnetic moment will prefer to be oriented along thelonger direction, because the magnetostatic energy of the device isminimized for this orientation of the ferromagnetic moment. Asillustrated in FIG. 7, the magnetic moment direction of layer 132 canalso be fixed by using the exchange bias field provided by anantiferromagnetic layer 140 in direct contact with layer 132. Theantiferromagnetic layer can be formed from various antiferromagneticmetals including Ir—Mn and Pt—Mn alloys. The antiferromagnetic layer canbe covered with a capping layer 145 to prevent corrosion of the layer140 and also for the purpose for forming a contact to the spin injectordevice 190. The capping layer 145 can be formed from TaN, Ta, Ru, Cu andother metals.

The magnetic moment of the ferromagnetic layer 132 can also be pinned orfixed in a particular direction by forming the ferromagnetic layer froma magnetically hard magnetic material such as an alloy of Co and Fe withPt or Cr.

While the ferromagnetic layer is preferred to be in direct contact withthe MgO tunnel barrier 123, it is also possible to separate theferromagnetic electrode 132 and the tunnel barrier 123 by a thin spacerlayer, providing that the spacer layer does not significantly diminishthe spin polarization of the electrons injected from the ferromagneticlayer 132 into the semiconductor layer 115 through the tunnel barrier123. A spin injector device 190 a of this type is illustrated in FIG. 9where the spacer layer is shown as layer 225. As described in U.S. Pat.No. 5,764,567 to Parkin with reference to magnetic tunnel junctiondevices formed with alumina tunnel barriers, the ferromagneticelectrodes in such MTJs can be separated from the tunnel barrier by oneor more thin spacer layers 225 formed from Cu and other non-magneticmetallic materials while maintaining significant tunnelingmagnetoresistance. Likewise, the tunnel barrier 123 may be separatedfrom the semiconductor layer 115 by a spacer layer 227. The types ofnon-magnetic metallic materials which are preferred are those whichdisplay large values of giant magnetoresistance in metallic spin-valvestructures or in metallic magnetic multilayers. These include Ag and Auas well as Cu.

In the embodiments shown herein, the semiconductor LED 111 serves as anoptical detector of the spin injection efficiency, but is not essentialfor the spin injection process. The semiconducting layer 115 can becomprised of a wide range of semiconductors in addition to GaAs,including the families of GaAs related semiconductors, Al_(1-x)Ga_(x)As(in which x can be varied between 0 and 100%) and In_(y)Ga_(1-y)As (inwhich y can be varied between 0 and 100%), the family of II-VIsemiconductors such as ZnSe, nitrides including GaN, InGaN and GaNInAs,antimonides including GaSb and InGaSb, phosphides including InP andInGaP, and Si and related compounds including Ge and SiGe, andheterostructures of these compounds.

While the spin injector, which includes the ferromagnetic layer 132 andthe MgO tunnel barrier 123, is shown attached to the surface of thesemiconducting layer 115 in FIGS. 2B and 7-9, the spin injector can alsobe attached to the side or edge of a semiconducting structure orheterostructure.

The spin injector device 190 shown in FIG. 7 is a source ofspin-polarized electrons and is operated by applying a voltage acrossthe device (by attaching electrical leads between the top of the deviceand the semiconductor substrate) so that electrons (or holes) flow fromthe ferromagnetic layer 132 across the tunnel barrier 123 into thesemiconducting layer 115. (Note that electrical current, by convention,flows in the direction opposite to that of the flow of electrons.) Theremay be non-magnetic spacer layers 225 and 227 on one or both sides ofthe MgO tunnel barrier 123 as shown in FIG. 9. The direction of the flowof electrons is indicated by the direction of the arrow 181 (theelectrical current flows in the opposite direction). The spin injectormay be one component of a semiconducting spintronic device. Within thesemiconducting part of the device, which may be comprised of amultiplicity of semiconducting materials, the direction of the spin ofthe flowing electrons may be changed by the application of electric ormagnetic fields. The magnetic fields may be provided by various meansincluding creating an Oersted field by passing current throughneighboring electrical wires or regions of the device. Alternatively,magnetic fields can be created by creating magnetic domain walls inmagnetic wires or other magnetic structures, as described in Applicant'scopending, commonly owned application titled “System and Method forWriting to a Magnetic Shift Register” to S. S. P. Parkin, filed on Jun.10, 2003 (application Ser. No. 10/458,147), which is hereby incorporatedby reference in its entirety. Alternatively, magnetic fields can becreated from the demagnetizing fields emanating from the edges or sidesof patterned magnetic nano-elements or from inhomogenous magnetizationin magnetic nano-elements or wires. Electric fields may also be used tomodulate the spin-polarization of the injected spin-polarized electronsthrough spin-orbit interactions and may be applied by applying voltageto electrical contacts or gates situated on or close to the spintronicdevice.

A device of the same or similar structure to that of the spin injectorcan also be used as a detector or analyzer of spin-polarized electrons(or holes) by flowing these electrons (or holes) from the surface of thesemiconducting layer 115 through the MgO tunnel barrier 123 into theferromagnetic detection layer 132 as illustrated in FIG. 8 by the device190′ and by the direction of the flow of electrons given by arrow 182;the primed numerals in FIG. 8 (including 140′ and 145′) designatecomponents that are like their unprimed counterparts.

The conductance of the device 190′ depends on the orientation of thespin polarization of the electrons with respect to that of the magneticmoment of the ferromagnetic layer 132′ at the interface with the MgOlayer 123′. The conductance will be greater if there are available emptyelectronic states in the ferromagnetic 132′ with the same spinpolarization as that of the tunneling electrons. This is a manifestationof spin-dependent tunneling effect.

For useful spintronic devices, the spin injector must provide currentwhich is sufficiently spin-polarized on entry into the semiconductingportion of the device that the inevitable loss of spin polarization asthe charge carriers are manipulated within this portion of the devicedoes not prevent the manipulation and subsequent detection of the spinpolarization of the charged carriers. Thus, the spin injector preferablyinjects a current which is more than 20% spin polarized in thesemiconductor, more preferably more than 30% and still more preferablygreater than 40%, and still more preferably greater than 50% spinpolarized. This is readily accomplished with the MgO tunnel spininjector of this invention.

In 1990 Datta and Das [Applied Physics Letters 56, 665 (1990)] proposeda spintronic device comprised of two metallic ferromagnetic electrodesconnected to a two dimensional electron gas within a GaAs basedheterostructure. The ferromagnetic electrodes are used as spin injectorsand spin analyzers. Between the two ferromagnetic electrodes a gatecontact is used to apply an electric field so as to cause the spin ofelectrons injected from one contact to precess, because of spin-orbitcoupling, during their motion from the first to the second ferromagneticcontact. This device has not yet been successfully fabricated because,as yet, there has been no successful demonstration of significant spininjection from a metallic ferromagnetic contact into a semiconductor.The spin injector 190 and detector 190′ of the current invention basedon ferromagnetic layers 132 and 132′ in contact with MgO tunnel barriers123 and 123′ in contact with a semiconductor 115 enable construction ofthe Datta-Das spintronic device.

While the present invention has been particularly shown and describedwith reference to the preferred embodiments, it will be understood bythose skilled in the art that various changes in form and detail may bemade without departing from the spirit and scope of the invention.Accordingly, the disclosed invention is to be considered merely asillustrative and limited in scope only as specified in the appendedclaims.

1. A method, comprising: forming a MgO tunnel barrier on a surface of anunderlayer; forming an overlayer on the MgO tunnel barrier to constructa spintronic element, with one of the underlayer and the overlayerincluding a layer of semiconductor material, and the other one of theunderlayer and the overlayer including a layer of magnetic materialselected from the group consisting of ferromagnetic materials andferrimagnetic materials; and annealing the MgO tunnel barrier at atemperature selected to improve tunneling characteristics of the MgOtunnel barrier, wherein i) the MgO tunnel barrier is sandwiched betweenthe underlayer and the overlayer, and ii) the underlayer, the MgO tunnelbarrier, and the overlayer are configured to enable spin-polarizedcharge carrier transport between the layer of semiconductor material andthe layer of magnetic material, said forming a MgO tunnel barrierincluding: depositing Mg onto the surface of the underlayer to form a Mglayer thereon; and directing additional Mg, in the presence of oxygen,towards the Mg layer to form a MgO tunnel barrier in contact with theunderlayer, the oxygen reacting with the additional Mg and the Mg layer.2. The method of claim 1, wherein the thickness of the Mg layer isselected to be large enough to prevent oxidation of the underlayer butsmall enough that, upon reaction of the oxygen with the Mg layer,substantially all the Mg in the Mg layer is convened into MgO.
 3. Themethod of claim 1, wherein the Mg layer is deposited in the absence ofsubstantial amounts of reactive oxygen.
 4. The method of claim 1,wherein the MgO tunnel barrier has a thickness of between 3 and 50angstroms.
 5. The method of claim 1, wherein the MgO tunnel barrier andthe layer of semiconductor material are formed so that they arelattice-matched, and each of the MgO tunnel barrier and the layer ofsemiconductor material is crystalline.
 6. The method of claim 1, whereinthe semiconductor material includes GaAs, the MgO tunnel barrier issubstantially (100) oriented, and the magnetic material includesferromagnetic material that is bcc and substantially (100) oriented. 7.The method of claim 1, wherein the semiconductor material includes Si.8. The method of claim 1, wherein upon application of a voltage acrossthe spintronic element, the spin polarization of charge carriers flowingbetween the MgO tunnel barrier and the layer of semiconductor materialis greater than 20%.
 9. The method of claim 1, wherein upon applicationof a voltage across the spintronic element, the spin polarization ofcharge carriers flowing between the MgO tunnel barrier and the layer ofsemiconductor material is greater than 40%.
 10. The method of claim 1,wherein upon application of a voltage across the spintronic element, thespin polarization of charge carriers flowing between the MgO tunnelbarrier and the layer of semiconductor material is greater than 50%. 11.The method of claim 1, wherein the annealing temperature is at least300° C.
 12. The method of claim 1, wherein the annealing step includesannealing the MgO tunnel barrier at a temperature of at least 300° C.for 60 minutes.
 13. The method of claim 1, wherein the magnetic materialincludes ferromagnetic material.
 14. The method of claim 13, wherein theferromagnetic material includes an alloy of Co and Fe.
 15. The method ofclaim 1, further comprising: forming a first lead that is in electricalcommunication with the semiconductor material; and forming a second leadthat is in electrical communication with the magnetic material, therebyenabling at least one of the following: the flowing of spin-polarizedcharge carriers into the spintronic element, and the flowing ofspin-polarized charge carriers out of the spintronic element.
 16. Themethod of claim 1, further comprising: forming a second spintronicelement, the second spintronic element being in electronic communicationwith the spintronic element of claim 1, the spintronic elements togetherforming respective devices for spin injection and spin detection. 17.The method of claim 1, wherein the semiconductor material is selectedfrom the group consisting of Al_(x)Ga_(1-x)As, In_(y)Ga_(1-y)As, ZnSe,GaN, InGaN, GaNInAs, GaSb, InGaSb, InP, InGaP, Si, Ge, SiGe, andheterostructures thereof, in which x and y are between 0 and
 1. 18. Themethod of claim 1, comprising annealing the spintronic element toincrease the spin polarization of charge carriers passed through theelement.
 19. The method of claim 1, wherein each of the MgO tunnelbarrier and the layer of semiconductor material is crystalline.
 20. Themethod of claim 19, wherein upon application of a voltage across thespintronic element, the spin polarization of charge carriers flowingbetween the MgO tunnel barrier and the layer of semiconductor materialis greater than 20%.
 21. The method of claim 20, wherein thesemiconductor material includes GaAs.
 22. The method of claim 20,wherein the semiconductor material includes Si.
 23. The method of claim20, wherein the MgO tunnel barrier is substantially (100) oriented, andthe magnetic material includes ferromagnetic material that is bcc andsubstantially (100) oriented.
 24. The method of claim 20, wherein theMgO tunnel barrier is in direct contact with both the layer ofsemiconductor material and the layer of magnetic material.
 25. Themethod of claim 20, further comprising: forming a first lead that is inelectrical communication with the semiconductor material; and forming asecond lead that is in electrical communication with the magneticmaterial, thereby enabling at least one of the following: the flowing ofspin-polarized charge carriers into the spintronic element, and theflowing of spin-polarized charge carriers out of the spintronic element.